1. Field of the Invention
The present invention relates to a detection circuit for envelope-detecting of a high frequency signal used in a communications device sending and receiving high frequency signals and a measuring device measuring signal levels of high frequency signals.
2. Description of the Related Art
An example of the configuration of a conventional high frequency detection circuit is shown in FIG. 12. As illustrated, the detection circuit is constituted by a diode D1, a capacitor C1, a resistor R1 and a direct current (DC) bias circuit formed by a fixed voltage source VD.
The diode D1 is used as an active element. The anode of the diode D1 is connected to an input terminal RFin of high frequency signals, while the cathode is connected to the output terminal of the detection circuit. Furthermore, the DC bias circuit providing a DC bias voltage Vd is connected to the anode of the diode D1 through the resistor R1, while the capacitor C1 for eliminating the high frequency component is connected to the cathode of the diode D1. Note that, as illustrated, a load resistance R.sub.L is connected to the output terminal of the detection circuit.
A high frequency signal is input to the input terminal RFin. By the rectifying effect of the diode D1 and the capacitor C1 with a sufficiently large capacitance, a voltage signal in accordance with the envelope of the input high frequency signal is output as a detection output signal Vout.
In the high frequency power detection circuit, it is required that a linear detection output signal Vout can be obtained from as low a signal level as possible to as high a signal level as possible, that is, in a wide dynamic range.
FIG. 13 shows an example of a detection characteristic of a high frequency power detection circuit using a high frequency diode as an active element. In this example, a silicon Schottky barrier diode is used and the DC bias voltage Vd of the diode D1 in FIG. 12 is set to 0V (zero bias). The graph depicts a relation between the input high frequency power Pin and the output voltage Vout obtained when the frequency of the high frequency signal is 10 GHz.
A silicon Schottky diode has mainly been used for high frequency power detection circuits because of its small turn-on voltage. Since the threshold voltage is small, the minimum detectable level of the input power becomes low.
By supplying a DC bias voltage Vd to the diode (that is, non-zero bias), it is possible to make the minimum detectable level of input power smaller, however, at the same time, the minimum detectable level of the input power rises, the current consumption increases, and the noise increases since the DC offset voltage rises.
A detection circuit using a silicon Schottky diode has a so-called hybrid configuration. Namely, diodes are mounted on a dielectric substrate such as a ceramic substrate, while passive elements such as resistors and capacitors are soldered to the dielectric substrate or formed on the dielectric substrate. The diodes and other devices are connected by wire bonding and the like.
In the conventional detection circuit using a silicon diode described above, however, since hybrid technology is used, the operational frequency band width is limited by influence of the parasitic inductance of the wire bonding. Further, since there are a large number of the manufacturing steps, problems such as a rise in the manufacturing costs and an increase of fluctuation in manufacture are caused. One means to solve these problems is monolithication. However, a silicon diode is not suited for monolithication. Further, in a radio communications device where there are strict requirements for miniaturization of the components, integrated circuits are being increasingly used in the high frequency circuit portions as well. Since the integration of the detection circuit using a silicon diode with other high frequency circuits is difficult, at the present time high frequency devices with signal frequencies higher than 1 GHz mostly use gallium arsenide (GaAs) semiconductors.
The problems mentioned above can be solved by using GaAs diodes. However, the threshold voltage of a GaAs diode is higher than that of a silicon diode. For this reason, the minimum detectable level of input power is large in a high frequency power detection circuit using a GaAs diode. In this way, a GaAs diode is better than a silicon diode in the point of monolithication, but is inferior in terms of the detection performance.
One means to improve the detection performance is to make the current-voltage characteristic (IV characteristic) of the diode more linear. However, the IV characteristic of a diode becomes a logarithmic function of the voltage between the anode and cathode of the diode regardless of the type of the diode and the semiconductor material. Accordingly, it is theoretically difficult to make the IV characteristic of the diode close to linear.
The present invention was made in consideration with such a circumstance and has as an object thereof to provide a high frequency power detection circuit constituting a monolithic high frequency power detection circuit by a GaAs semiconductor and thereby capable of realizing a small sized, low cost, and broad band detection circuit, capable of suppressing variations in the detection characteristics due to variations in a pinchoff voltage of the field effect transistors, and capable of obtaining a high detection output voltage at a low power of a high frequency signal.